216 



action of the elementary particles of the rock while passing into a 

 solid state. 



Jointed structure is also discussed at some length : and joints are 

 divided into four classes, called dip, strike, diagonal, and tabular ]omX,%. 

 The two former are most constant, often highly inclined, and divide 

 the slaty beds into great rhombohedral masses. They are supposed 

 to have been formed by mechanical tension while the rocks passed 

 into a solid state. Cleavage planes are not parallel to them ; and they 

 cannot arise from molecular or true crystalline action ; because, as 

 shown from examples in North Wales, they cut through the pebbles 

 of beds of conglomerate : sometimes however, among the accidents 

 of structure, true cleavage planes and joints become confounded. 



Bala limestone. — System of the Berwyns. — Fossiliferous beds on the 

 line of the Holyhead road, &;c. 



This limestone ranges through Cader Dinmael to Glyn Diffwys on 

 the Holyhead road, a few miles east of Corwen. Thence it is pro- 

 longed towards the south ; but its continuity is broken, and for some 

 miles its range has not yet been made out. Exactly on the line of strike 

 (N.N.E. and S.S.W.) it breaks out again near Bala, and ranges 

 thence to the neighbourhood of Dinas Mawddwy, dipping steadily 

 to the E.S.E. 



From this limestone there is an ascending section to the very crest 

 of the Berwyn chain, south of the road from Bala to Llangynog. 

 In this ascending section are higher calcareous slates, which in one 

 or two places have been burnt for lime. But on the east fiank of this 

 part of the chain there is a synclinal line, beyond which for several 

 miles the beds dip to the N.W.; and a series of slate rocks, alternating 

 with a few bands of porphyry, are again brought up to the surface. 

 Some of these are only a repetition of a portion of the slates and 

 porphyries on the east side of the range of the Bala limestone. These 

 older rocks abut against, and, in consequence of enormous convul- 

 sions, in one or two places seem to overlie the Silurian rocks (among 

 the tributaries of the Severn) described by Mr. Murchison. 



The synclinal line above noticed appears to strike about N.N.E. ; 

 but the mean direction of the water-shed of the Berwyns is about 

 N.E. : hence the synclinal line, and the calcareous slates overlying 

 the Bala limestone, are, near the Llangynog road, brought to the west 

 side of the chain ; and the crest of the ridge, extending beyond Ca- 

 der Ferwyn, is composed of the older slates and porphyries dipping 

 towards the N.W, Still further north, either from a great flexure 

 or (more probably) from an enormous fault, we have for several 

 miles a great series of beds dipping to a point a few degrees east of 

 north. They alternate here and there with porphyry, contain many 

 fossils, and in their highest portion near Llansaintfraid Glyn Ceiriog, 

 contain two bands of limestone. At this place they seem to form a 

 regular ascending section, conducting without a break to the over- 

 lying Upper Silurian flagstone. 



The author then notices the undulating country east of the Ber- 

 wyns, extending from the Severn, near Pool, through the ramifica- 



